Recent developments in microfabrication techniques have permitted the integration of microminiature tools for biochemical analysis within a tiny microfluidic device. Microfluidic devices have been successfully demonstrated as useful for a wide variety of separation techniques as well as for sample pretreatment and/or manipulation methods. Complete chemical processing systems, e.g., reaction chambers, separation capillaries and their associated electrode reservoirs, as well as certain types of detectors, can be consolidated on a microfluidic device or microchip. Such “labs-on-a-chip,” in principle, permit effective utilization and manipulation of minute quantities of material. Systems have been developed that permit the efficient transfer of nanoliter quantities or other small quantities of a fluid sample from the spatially concentrated environment of a microfluidic device to “off-chip” analytical or collection devices without an increase in sample volume. (See, U.S. Pat. No. 5,872,010, the whole of which is hereby incorporated by reference herein.) However, methods for moving small quantities of samples from a sample vial or holder onto a chip or directly to an analytical device without either evaporation or dilution are less well developed.
One important issue for the further development of microfluidic devices is that sample introduction into a device and subsequent injection of a sample aliquot into, e.g., a separation channel or reaction compartment on the device must be carried out in a reliable, accurate and convenient manner. Moreover, the method should not result in any sample “cross talk” and should be easy to automate. This is very difficult to accomplish when multiple channels on one microfluidic device are used.